1. Technical Field
The present invention relates to a buried semiconductor laser and a method for manufacturing the same. Specifically, the present invention relates to a structure of a p-type gallium indium arsenide (InGaAs) contact layer in a buried semiconductor laser of group III-V element, employed for optical communications, and a method for manufacturing the same.
2. Related Art
In an optical transmission system that achieves a communication with higher capacity and higher rate, semiconductor lasers employed as light sources are mainly composed of indium phosphide (InP)-containing, group III-V compound semiconductors. Lower threshold current and higher efficiency are required for an active layer in a semiconductor laser, so that a strained quantum-well structure is used in order to improve such characteristics. For example, a technology for achieving a lower threshold current by applying compressive strain of 1.53% to a well layer is disclosed in Electronics Letters, vol. 26 (1990), p.p. 465-467. However, when a strain larger than a critical strain is applied to a quantum well layer, a defect such as dislocation is generated in an emission layer, considerably deteriorating laser characteristics. To solve the problem, Japanese Patent Laid-Open No. H4-22,185 (1996) discloses a semiconductor optical device having a quantum well layer composed of a multiple-layered structure with compressive strain layers and tensile strain layers, so that an average strain is reduced to a level of not higher than a threshold strain, thereby achieving an improved performance of a semiconductor optical device.
Since a larger strain to the active layer is applied to the semiconductor devices described above at a level up to the limit of the material property, the devices are structurally vulnerable by a stress generated in the whole laser device. In general, a buried structure, which is manufactured by forming pn-buried type or higher resistance buried-type current-blocking structures are formed on both sides of the active layer and then filling the whole current-blocking structures with an InP clad layer, is also employed in order to reduce a leakage current from the semiconductor laser around the active layer at wider range of temperatures to achieve a faster operation of the device. In such buried layer, it is required to control a strain for the purpose of avoiding a generation of a dislocation extending to the active layer.
In a semiconductor light emission device described in Japanese Patent Laid-Open No. H11-87,764 (1999), a multiple-layered semiconductor layer including a buffer layer, a first clad layer, an active layer, a second clad layer and a cap layer is formed on an n-InP compound semiconductor substrate. The semiconductor light emission device described in Japanese Patent Laid-Open No. H11-87,764 is configured that at least one of a buffer layer and a cap layer has a tensile strain, so that improvements in device characteristics such as reduction in threshold current would be achieved.
On the other hand, a semiconductor laser device having a contact layer is disclosed in Japanese Patent Laid-Open No. 2004-95,975. The semiconductor laser device disclosed in Japanese Patent Laid-Open No. 2004-95,975 is designed that a different strain level between in a first active layer and in a second active layer, so as to suitably control a gain peak wave-length. The semiconductor laser device described in Japanese Patent Laid-Open No. 2004-95,975 is provided with a strain adjustment film to control a strain in the active layer. A growth of an InGaAs contact layer described in Japanese Patent Laid-Open No. 2004-95,975 is conducted concurrently with a growth of the active layer, which achieves a flat geometry. Since a strain in the active layer is created by utilizing film stresses generated in an insulating film and an electrode film disposed on the InGaAs contact layer in Japanese Patent Laid-Open No. 2004-95,975, the InGaAs contact layer does not exhibit a strain.
In a structure of a buried semiconductor laser employing an n-type InP substrate, a p-type InGaAs contact layer having a carrier density of 1×1019 cm−3 or higher is required to be disposed between a p-side metallic electrode and a p-type InP clad layer, for reducing a driving current of semiconductor laser. Since such structure of the buried semiconductor laser employing the n-type InP substrate is configured that the pnp buried layers or the high resistance buried layers are selectively grown on both sides of the active layer, the whole structure is filled with the p-type InP clad layer, and then the p-type InGaAs contact layer is grown, the portion of the resultant contact layer immediately above the active layer is not completely flat and an inclined portion is remained.
When the InGaAs contact layer is in lattice match with InP at a flat section that is sufficiently remote by 50 μm or further from the inclined portion of the p-type InGaAs contact layer immediately above the active layer in such structure of the buried semiconductor laser employing the n-type InP substrate, a tensile strain toward the InP clad layer is applied at the inclined portion of the p-type InGaAs contact layer immediately above the active layer. Such tensile strain generated at the inclined portion of the p-type InGaAs contact layer immediately above the active layer may possibly induce a generation of a defect such as dislocation and the like, causing a concern for adversely affecting a reliability of the semiconductor device.
According to detailed investigations by the present inventors, it was found in such structure of the buried semiconductor laser employing the n-type InP substrate that, although the p-type InGaAs contact layer in the flat section, which is sufficiently remote by 50 μm or further from the inclined portion immediately above the active layer, is in a condition of lattice match on the InP substrate, a larger tensile strain of about 0.3 to 0.4% at a maximum is applied to the inclined portion immediately above the active layer. It is considered that the reason is a difference in uptake efficiency for atomic In and atomic Ga between the inclined portion and the flat portion. In general, the atomic uptake efficiency between the inclined portion and the flat portion is varied somewhat by growth conditions of the p-type InGaAs layer, but it is difficult to obtain the same composition for the inclined portion and for the flat portion. A tensile strain in the portion of the p-type InGaAs contact layer in the inclined portion immediately above the active layer may induce a generation of a defect such as dislocation and the like, leading to adversely affecting a reliability of the semiconductor device.
The present invention is made on the basis of the above-described circumstances, and is directed to a semiconductor device exhibiting an improved reliability by inhibiting a generation of a defect such as dislocation and the like.